linux-kernel - Re: [PATCH bpf-next] bpf: Add bpf_read_raw

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Message-ID: <C7F3F33B-4A8E-428C-9FED-FB635955C2B1@fb.com>
Date:   Fri, 26 Aug 2022 18:09:23 +0000
From:   Song Liu <songliubraving@...com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     Song Liu <song@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper



> On Aug 26, 2022, at 9:33 AM, Namhyung Kim <namhyung@...nel.org> wrote:
> 
> On Thu, Aug 25, 2022 at 10:53 PM Song Liu <song@...nel.org> wrote:
>> 
>> On Thu, Aug 25, 2022 at 10:22 PM Namhyung Kim <namhyung@...nel.org> wrote:
>>> 
>>> On Thu, Aug 25, 2022 at 7:35 PM Song Liu <songliubraving@...com> wrote:
>>>> Actually, since we are on this, can we make it more generic, and handle
>>>> all possible PERF_SAMPLE_* (in enum perf_event_sample_format)? Something
>>>> like:
>>>> 
>>>> long bpf_perf_event_read_sample(void *ctx, void *buf, u64 size, u64 flags);
>>>> 
>>>> WDYT Namhyung?
>>> 
>>> Do you mean reading the whole sample data at once?
>>> Then it needs to parse the sample data format properly
>>> which is non trivial due to a number of variable length
>>> fields like callchains and branch stack, etc.
>>> 
>>> Also I'm afraid I might need event configuration info
>>> other than sample data like attr.type, attr.config,
>>> attr.sample_type and so on.
>>> 
>>> Hmm.. maybe we can add it to the ctx directly like ctx.attr_type?
>> 
>> The user should have access to the perf_event_attr used to
>> create the event. This is also available in ctx->event->attr.
> 
> Do you mean from BPF?  I'd like to have a generic BPF program
> that can handle various filtering according to the command line
> arguments.  I'm not sure but it might do something differently
> for each event based on the attr settings.

Yeah, we can access perf_event_attr from BPF program. Note that
the ctx for perf_event bpf program is struct bpf_perf_event_data_kern:

SEC("perf_event")
int perf_e(struct bpf_perf_event_data_kern *ctx)
{	
	...
}

struct bpf_perf_event_data_kern {
        bpf_user_pt_regs_t *regs;
        struct perf_sample_data *data;
        struct perf_event *event;
};

Alternatively, we can also have bpf user space configure the BPF 
program via a few knobs. 

And actually, we can just read ctx->data and get the raw record, 
right..?

Thanks,
Song